Conventionally, adjustment of an angle of inclination of a swash plate in a variable displacement piston machine provided with a valve plate and a cylinder block with a plurality of pistons is done usually as is described in Japanese Patent Publication No. 522/1978. That is, fluid pressure controlled by a pressure compensator valve is caused to act on an operating plunger which is in contact with the swash plate, so that the movement of the operating plunger causes the swash plate to incline. The amount of the discharge and intake of the fluid is adjusted through the alteration in the stroke of the piston caused by the change in the angle of inclination of the swash plate. On the other hand, the present applicant has recently proposed and filed an application for utility model registration (Japanese Utility Model Application No. 61-37882) for an improved variable displacement piston machine that enables the angle of inclination of a swash plate to be adjusted by using pistons in a cylinder block, instead of an operating plunger.
In this improved variable displacement piston machine, as illustrated in FIG. 5, a valve plate has a high-pressure port H, a low-pressure port L and a control port P which is disposed between the high-pressure and low-pressure ports. The control port P is adapted to selectively communicate either to the high-pressure port H or to a tank T by a solenoid controlled valve SV. A proportional reducing valve DV is installed in a passage between the solenoid controlled valve SV and control port P. When the control port P is switched to communicate to the high-pressure port H, pressure on a secondary side of the reducing valve DV, which varies according to the pressure of the high-pressure port H, acts on the control port P and a piston hole A communicating with the control port as the cylinder block rotates, so that the swash plate is pressed towards a neutral position by means of a piston fitted in the piston hole A. Displacement of the swash plate is adjusted depending on the discharge pressure in case of a pump, and the intake pressure in case of a motor, and the discharge quantity of the pump and the intake quantity of the motor are controlled to respective predetermined values relative to the discharge pressure of the pump and the intake pressure of the motor. Also, when the control port P is selectively connected to the tank T, the swash plate is made to return to its maximum angle of inclination by the action of a return spring (not shown in the drawing).
In regulating the angle of inclination of the swash plate in the improved variable displacement piston machine, however, there is a drawback that because the cylinder block has an odd number (nine normally) of piston holes A, an inclining moment to act on the swash plate through the pistons fitted in these piston holes A varies due to the change of positions of the piston holes made while the cylinder block is rotating. The variation in the inclining moment makes the swash plate unsteady and makes the adjustment of the angle of inclination insecure, which adversely affects accurate control of the discharge and intake. Furthermore, as the proportional reducing valve DV is used in order to switch the solenoid controlled valve SV for returning the swash plate to the position of maximum angle of inclination from the neutral position, and vice versa, the speed at which the position of the swash plate is changed can be made somewhat slow depending on a selected reduction rate, compared with a case that the reducing valve is not provided. As a result, however, force to press the swash plate decreases in proportion to the pressure reduction rate selected, and this force becomes too small to overcome the change in the inclining moment. As a result, the unsteadiness or sway of the swash plate cannot be eliminated completely. Decreasing of the pressure reduction rate, however, causes the swash plate to change its position so quickly that a shock occurs.
Accordingly, the object of the present invention is to provide a variable displacement piston machine that prevents the sway of the swash plate due to the change in the inclining moment to realize stable control of the angle of inclination of the swash plate and that enables the moving speed of the swash plate to be controlled as desired through an operation carried out by an external device to eliminate the shock which occurs when the position of the swash plate is changed.